1. Field of the Invention
This invention relates to waste disposal, and to the disposal in particular of fly ash, produced in the burning of coal, and of aqueous sludges containing heavy metal species such as tungsten, cadmium, chromium, strontium, copper, iron, lead, nickel, zinc, molybdenum, tin, magnesium, aluminum, phosphorus, and manganese. The sludges also usually contain other inorganic ions such as alkali metal and alkaline earth cations and inorganic anions such as oxides, hydroxides, carbonates, silicates, sulfides and basic sulfates.
Fly ash comprises finely divided inorganic products of the combustion of coal. Enormous amounts of fly ash are produced annually nationwide, principally from burning coal in, for instance, power plants. Disposal of these amounts of fly ash poses an increasingly difficult problem because the volume, the particulate nature and the varied chemical composition of fly ash limit the number of acceptable disposal sites. Furthermore the capacity of acceptable disposal sites is not unlimited. It is desirable to be able to convert fly ash economically into a usable product rather than merely discarding it, but up to now it has been necessary to incur the cost of another product, such as cement, to combine with the fly ash as a binder.
Water-borne heavy metal sludges produced in plants which process or refine metals also present disposal problems. Environmental considerations limit the number of areas in which the sludges per se can be disposed of, principally because of the risk that the heavy metals in the sludge will leach out into the surrounding soils and will be carried into surface and subsurface waters. For this reason, conventional landfills are no longer acceptable. There is, therefore, a need for a technique that can immobilize or fix the heavy metals in the sludges in a useful, long-lasting product which presents a minimal risk that the heavy metal species will subsequently leach out of the product into the surrounding environment. The present invention is a technique which accomplishes this desired goal while simultaneously achieving the desired goal of permitting the conversion of coal fly ash into a useful product.
3. Description of the Prior Art
Fly ash of particular composition has been used as an additive in Portland cement, but the fly ash has not itself been hydraulic (i.e. self-setting). Rather, the purpose of adding the fly ash has been to strengthen the cement, evidently by reacting with excess free lime and by reacting with sulfate compounds which would otherwise attack the tricalcium aluminate and tetracalcium aluminoferrite compounds of Portland cement.
While heavy metal wastes have been disposed of in solids formed by compounding the wastes with commercial Portland cement, this approach is undesirable because it is expensive in its consumption of cement.
Several techniques purporting to permit the fixation of wastes into stable products which are safe for disposal are commercially available, but are not believed to offer the range of applicability nor the economies of operation available in the present invention. In each of these techniques, sludge is mixed with other ingredients and the mixture is compacted to a given density to form a dense body. One such technique, known as "Chemfix" (sold through Chemfix, Inc.), uses a combination of sodium silicates and silicate cement setting agents which react with polyvalent metal ions to form a cross-linked structure which holds the metal ions. In two other techniques, the reactions by which heavy metal species are fixed in the compacted body occur very slowly, e.g. on the order of 60 days. Another technique employs a proprietary reagent known as "Calcilox" (sold through Dravo Corp.) which is added together with large amounts of lime to the sludges under treatment. A third technique, termed "Pozotec" (sold through I.U. Conversion Systems, Inc.) forms a compacted product from a combination of fly ash, lime, and sulfate sludge from sulfur dioxide scrubbing units.
The present invention is preferable to each of these techniques because it can be carried out quickly; a solid body in which the heavy metal species are fixed is formed in a matter of minutes. Also, the present invention uses only the waste materials, namely fly ash and metal sludges, and thus does not require the addition of other reagents which would add to the expense and the complexity of the waste disposal operation.